Code division multiple access systems such as direct sequence (DS-CDMA) communication systems have been proposed for use in cellular telephone systems operating at 800 MHz and in the personal communication system (PCS) frequency band at 1800 MHz. In a DS-CDMA system, all base stations in all cells may use the same radio frequency for communication. Base stations are uniquely identified in the system by uniquely-assigned spreading codes. Two specified pseudorandom noise (PN) sequences of 2.sup.15 bits length are used by all the base stations. In a quadrature modulated system, one sequence is used for the in-phase (I) channel spreading of the I channel symbols and the other is used for the quadrature (Q) channel spreading of the Q channel symbols. Mobile stations in the system possess the same two 2.sup.15 bits length spreading codes and use them for the initial de-spread of the I and Q channels.
Before the spreading on the I and Q channels, the symbols for transmission are spread using a process known as Walsh covering. When in a call, each mobile station is assigned a unique Walsh code by the base site to ensure that transmission to each mobile station within a given cell is orthogonal to transmission to every other mobile station, assuming that a different Walsh code is used for each mobile station. In this manner, traffic channels are established for two-way communication between a base station and a mobile station.
In addition to traffic channels, each base station broadcasts a pilot channel, a synchronization channel, and a paging channel. The pilot channel is formed by a constant level signal that is covered by Walsh code 0, which consists of all the same bits. The pilot channel is commonly received by all mobile stations within range and is used by the mobile station for: identifying the presence of a CDMA system, initial system acquisition, idle mode hand-off, identification of initial and delayed rays of communicating and interfering base stations, and for coherent demodulation of the synchronization, paging, and traffic channels. At the mobile station, the received RF signals include pilot, synchronization, paging, and traffic channels from all nearby base stations.
Referring to FIG. 1, a typical process for a mobile station to receive incoming calls is shown. At the start 10, a mobile unit will power up 12 various circuitry to complete a call. This includes powering up the RF portions of the mobile unit, the receiver circuits including a receiver search, and a digital signal processor (DSP) including a call processor as are known in the art. Once the mobile is powered up, the mobile unit proceeds to acquire 14 the pilot channel from the base station. Once the pilot channel is acquired the mobile unit will acquire synchronization 16 which aligns the timing of the mobile unit with the base station. The mobile station synchronizes to the base station by correlation to a unique Walsh code on the synchronization channel. Typically, mobile stations use a correlator as a receiver pilot searching element to serially search for the PN phases of the receivable pilots. Knowledge of the correct I and Q channel spreading PN phases of the base station(s) with which the mobile station communicates allows the coherent detection of all the other code channels transmitted by the base station.
Once the mobile is synchronized with the base station, the mobile will monitor the paging channels either continuously or intermittently (slotted mode) to see if there is any incoming call activity for the mobile. Monitoring the paging channels includes search a paging channel 18 to see if a paging message for the particular mobile unit is present 20. If no message is present, the mobile unit will check if it has searched all available paging channels 22. If not, the mobile unit will change RF frequency to tune to another paging channel 24. The mobile unit will search all the available paging channels in this way. If no paging message is found the mobile unite will power down 26 its RF portions, the receiver circuits including the receiver search, and the digital signal processor (DSP) including the call processor and return to an idle or sleep state until it is time again to check for paging messages. If a paging message is eventually found the mobile and base station will set up and connect to a traffic channel 28 for transmitting and receiving the call indicated by the paging message to connect to the base station. At this time the mobile unit and base station proceeds with the transfer of information (voice, data, etc.) until the communication is completed 30. Upon completion, the mobile unit powers down its circuitry 26 as before and returns to an idle state.
In accordance with the above procedure, three different channels (pilot, synchronizing, paging) are needed to be monitored to see if there is an incoming call waiting for the mobile. Unfortunately, acquiring all three channels takes time and power, and may not always be successful. In addition, the mobile unit must identify all the pilot signals that are receivable including the pilot signal from the base station with the strongest pilot channel.
The prior art pilot channel searching method creates further limitations for all of the other uses of the pilot channel after initial system acquisition. Typical DS-CDMA mobile station receivers utilize a rake receiver having three or more independently controlled fingers which are time aligned to the correct PN sequence phases using knowledge of the pilot channel phases determined by the receiver pilot phase searching element. The rake fingers are normally assigned to the strongest rays received from all communicating base stations as determined by the receiver pilot phase searching element. Ray assignments are updated in a maintenance process using the pilot phase searching element information. If the pilot phase searching element is slow, resulting in slow maintenance of the assignment of the strongest rays to the rake fingers, the receiving performance of the mobile station is degraded under fading conditions.
Idle hand-off is the process of attaching to and listening to the paging channel of the base station with the strongest pilot as identified by the pilot searching element. When the mobile station receives a page or accesses the system to place a call, it is important that the mobile station is listening to the page from, or tries to access, the base station associated with the strongest received pilot. This requires a fast pilot phase searching element, particularly when the mobile station is in motion.
A portable station may have to search the possible phase space of as many as twenty base stations every time it wakes up. To reliably receive the paging slot after waking up, the portable station must be listening to the base station which is providing adequate signal strength. When the mobile station is in motion, the correct base station to decode can easily change from one paging interval to the next paging interval. Therefore it is very important to have a fast pilot searching mechanism to identify the correct base station pilot before the start of the assigned paging slot.
For battery powered portable mobile stations it is also very important to conserve battery charge when waiting for pages. DS-CDMA provides a slotted mode that allows portable stations to power down except for the periods when their assigned paging slot information is transmitted by the base stations. The paging slot interval can be as short as 1.28 seconds and periods of 1.28 seconds multiplied by powers of two for more battery savings. During these intervals, the mobile station "sleeps" in a low power mode. However, using the prior art pilot searching mechanism requires the portable station to wake up well before the paging slot to allow sufficient time to sequentially search the PN sequence phase space. This negates a substantial part of the potential battery savings afforded by slotted mode.
There has been a proposal (for TIA/EIA interim standard IS-95C) for an addition Walsh channel, in addition to the pilot, paging, synchronization, and traffic channels, dedicated for paging activity. However, this still requires the mobile to acquire multiple channels.
Accordingly, there is a need for an apparatus and method for a mobile unit that avoids the problems associated with acquiring multiple channels to see if an incoming call is present. There is also a need to improve on battery savings of a mobile while providing simpler operation. It would also be an advantage to eliminate some of the problems associated with: pilot searching, maintaining system acquisition, soft hand-off, and slotted mode timing and operation.